Semiconductor solar cells are well known for transforming light into electric current. The efficiency of solar cells is limited in part by ohmic losses, which are affected by the dopant diffusion and contact screen printing used to fabricate the solar cells.
FIG. 1 shows a prior art solar cell 100. The solar cell 100 converts light striking photo-receptive regions 135 on its top surface into electric current, which can be transmitted to a load 150. The solar cell 100 includes an n-type emitter layer 115 overlying a p-type substrate 110, thereby defining a p-n junction 111. The emitter layer 115 contains highly doped n-wells 117 that form gridlines and can be covered with an anti-reflective coating (ARC) 120. Metallic fingers 125 are formed on top of the n-wells 117 to couple the n-wells to a busbar 130. The busbar 130 is coupled to the load 150, which in turn is coupled to a metallic contact 140 on a backside of the substrate 110.
The emitter layer 115 is formed by exposing the substrate 110 to a source of n-type ions, which then diffuse into a top surface of the base 100. The doping profile of the solar cell 100 has several drawbacks.
First, producing this profile results in excess un-activated dopants near the top surface, as the dopants are driven into the bulk of the substrate 100. This effect leads to varying levels of light absorption, the creation of electron-hole pairs, and unwanted recombination of electron-hole pairs. This is known as “dead layer,” in which blue light is not absorbed close to the top surface of the photo-receptive regions 135. Because of the high doping level near the surface, electron-hole pairs created in the dead layer quickly recombine before they can generate any current flow. Facetting, used to reduce the amount of light reflected from the solar cell before it can generate current.
Second, diffusion techniques used to form a conventional profile are not optimal for the formation of selective doping regions with a homogenous high resistivity photo-receptive region and low-resistance regions for gridlines, contact fingers, busbars, metal-silicon interfaces, and backside metallization.
Third, direct overlay of metal on the semiconductor can result in different work functions at the interface between the conductive fingers 125 and the emitter layer 115. To better match the work functions between a metal contact and the doped silicon, some prior art techniques melt the contacts 125 to form a silicide at the interface. While forming a silicide may help tailor the work functions, there are still undesirable ohmic losses and the potential of metal shunting.
Finally, lateral positioning of dopants across a substrate is becoming difficult as the line widths and wafer thicknesses are decreasing. Geometries in solar cell gridlines are expected to drop from about 200 microns to 50 microns, and later to drop even smaller. Present screen-printing techniques are ill equipped to fabricate devices with such small displacements. Moreover, as wafers are getting ever thinner, vertical and batch diffusion and screen printing become extremely difficult.